I had a HD555, it was the first nice headphones I bought. I bought a HD600, never regret buying it, sound was better and more detailed. Probably I'll buy a HD800 one day, and a nice amplifier for it. That is above the limit of spending for most people. I guess if I would have more money, I would buy the Orpheus.
Most people here dont realise that not everything sound related is snake oil. There are good reason to have balanced cables for headphones or have planar speakers or in this case tubes.
These are planar headphones, they dont have coils on the speaker. They work differently. Since there are no coils, which are heavy, the moving part is lighter, and so the frequency it can move is greater. Also, they need higher voltage than the usual headphones. So you either throw together a BJT/FET based amplifier, or you use tubes, since there arent many opamps for high voltage. They choose tubes, since they work. You will notice, there isnt the usual transformer on the output, it is not the bullshit amplifier.
The rest is just for the show. Price included. So what? Some people spend more on their cars.